Precise spectroscopic interrogation of the resonance features of confined atomic samples can be employed to build atomic frequency standards. As the need for compact, low-power frequency standards has grown, there has been increased emphasis on reducing the size, complexity, and cost of such devices. In particular, the development of reliable, low-power diode lasers has enabled the development of compact, portable atomic instrumentation.
The technology of gas cell atomic frequency standards is well known (see J. Vanier and C. Audoin, The Quantum Physics of Atomic Frequency Standards, Adam Hilger, Bristol and Philadelphia, 1989, ISBN 0-85274-434-X), and such devices have been in widespread use for many years. The existing art employs a vapor cell containing an alkali metal, typically rubidium or cesium, along with a buffer gas (see R. H. Dicke, “The Effect of Collisions Upon the Doppler Width of Spectral Lines,” Phys. Rev. Lett. Vol. 89, pp. 472–473, 1953), which is illuminated by either a discharge lamp or laser diode. The cell's optical transmission displays a narrow resonance feature when it is placed within a microwave cavity and energy is applied at the ground state hyperfine frequency. This response serves as a discriminator to lock the frequency of the microwave source, thereby producing a stabilized output.
In another embodiment, employing the technique of coherent population trapping (CPT), the microwave cavity is eliminated and instead the laser diode is modulated at one-half of the atomic hyperfine frequency (see N. Cyr, M. Têtu, and M. Breton, “All-Optical Microwave Frequency Standard: A Proposal,” IEEE Trans. Instrum. Meas., Vol. 42, No. 2, pp. 640–649, April 1993; J. Kitching, S. Knappe, N. Vuki{hacek over (c)}ević, R. Wynands and W. Weidmann, “A Microwave Frequency Reference Based on VCSEL-Driven Dark-Line Resonances in Cs Vapor,” IEEE Trans. Instrum. Meas., Vol. 49, No. 6, pp. 1313–1317, Dec. 2000; and J. Vanier, M. Levine, D. Janssen and M. Delaney, “The Coherent Population Trapping Passive Frequency Standard,” IEEE Trans. Instrum. Meas., Vol. 52, No. 2, pp. 258–262, April 2003), providing an all-optical device that is much smaller, lower in power, and less expensive to produce. (See S. Knappe, V. Velichansky, H. Robinson, J. Kitching, L. Hollberg, “Atomic Vapor Cells for Miniature Frequency References,” Proc. 17th European Frequency and Time Forum and 2003 IEEE International Frequency Control Symposium, May 2003). In particular, the gas cell can be made using the batch processing methods of micro-machining and micro-electromechanical systems (MEMS) technology. (See U.S. Patent Application No. 2002/0163394, L. Hollberg et al., “Miniature Frequency Standard Based on All Optical Excitation and a Micro-Machined Containment Vessel,” Nov. 7, 2002). As the size of the contained volume is reduced, it becomes increasingly necessary that the auxiliary components, including the optical source and detector, heaters and temperature control, and microwave assemblies, be reduced in size and complexity as well.
U.S. Pat. Nos. 6,265,945 and 6,320,472, along with “A Microwave Frequency Reference Based on VCSEL-Driven Dark Line Resonance in Cs Vapor,” by J. Kitching et al., and “The Coherent Population Trapping Passive Frequency Standard,” by J. Vanier et al. (more completely identified above) describe the principles of building an atomic frequency standard based on coherent population trapping (CPT). U.S. Patent Application No. 2002/0163394 and “Atomic Vapor Cells for Miniature Frequency References” by S. Kappe et al. (identified above) describe the design and fabrication of miniature gas cells for use in such an atomic frequency standard. U.S. Pat. No. 5,327,105 describes a small gas cell that contains a reflective optical path, with a mirror and beam splitter in a common optical configuration for coupling to a photodetector. U.S. Pat. No. 6,353,225 is representative of prior art devices in centimeter size dimensions that employ a laser diode and photodetector to measure the transmission of light through a gas, sensing its response at a particular atomic or molecular absorption line. The disclosed apparatus includes a separate laser diode and separate photodetector located in the same plane and a mirror.
Notwithstanding the developments and prior apparatus described above, there remains a desire and need for more compact, less complex and less expensive atomic frequency standards.